Articles of different kinds are often ground and/or polished by vibrating them in the presence of a grinding and/or polishing material (referred to generically as polishing material). Customarily, a quantity of articles to be polished and a quantity of polishing material are held within a vibratory chamber (or container) of a vibrating device; and the vibrating device applies controlled vibration to the articles and the polishing material so as to provide rapid relative movement between the articles and the polishing material. As a result, the polishing material impacts upon the articles being finished and provides grinding and/or polishing of the articles depending on the polishing material used, i.e., grit size or abrasiveness.
The material used in the grinding and/or polishing applications referred to herein can consist of two grinding and/or polishing components. First, a particulate grinding/or polishing media (referred to generically as polishing media) is used which remains in the vibratory chamber and generally consists of an abrasive or burnishing agent. The polishing media has a relatively large size, e.g., 0.025 in. or larger, and may consist of one or more materials commonly known in the art such as are available from Washington Mills, in Massachusetts and Florida. Second, a liquid grinding and/or polishing fluid or slurry (referred to generically as polishing fluid or slurry) is used which circulates through the chamber; it includes a grinding and/or polishing agent (referred to generically as polishing agent) mixed with water or an oil. The grinding and/or polishing agent is generally a powder, e.g., measured in microns and may include one or more materials such as A1.sub.2 O.sub.3 or SiC available from suppliers such as Norton Abrasives, K.C. Abrasives in Kansas City and Microgrit. Thus, depending on the particular application, the polishing material may include either grinding agents or polishing agents along with the media.
A polishing slurry tends to provide a pronounced cutting, abrading or grinding action when the polishing slurry is newly introduced into a container of the vibrating device; however, after a period of time, the polishing slurry tends to provide less efficient cutting action. Thus, vibrating devices have been developed wherein fresh polishing slurry is continuously or periodically introduced into the container of the vibrating device, and spent polishing slurry (or discharge slurry) is simultaneously removed from the container.
Dwyer et al., U.S. Pat. No. 3,411,248, shows a vibrating device wherein an admixture of water and grinding and/or polishing agent is introduced into the container of the vibrating device on a substantially continuous basis, and wherein spent grinding and/or polishing agent is similarly cleaned away on a substantially continuous basis.
An alternative approach is shown by Roberts, U.S. Pat. No. 3,353,796. However, steel burnishing balls or beads, used as the polishing media, are retained in a work tub and are not introduced or drained away during the polishing process.
Problematically however, neither Dwyer et al. nor Roberts show a means for responding to changing conditions in the container or work tub, i.e., Dwyer et al. and Roberts show only "open-loop" control systems. For example, various factors such as polishing slurry viscosity and the amount of material worn away from the articles being polished can affect the outflow of polishing slurry from the container or work tub. Similarly, factors such as water pressure can affect the amount of polishing slurry flowing into the container or work tub. Thus, when one or more of these factors causes a change in the inflow (or outflow) of polishing slurry or fluid into the container, without a corresponding change in the outflow (or inflow) from the container, the level of liquid in the container will increase or decease correspondingly. Disadvantageously, if this phenomenon continues, which it is likely to do over an extended period, and corrective steps are not taken by a human attendant, the container will either overflow or contain a less than adequate level of polishing slurry to effectively grind or polish the articles. In either case the desired grinding or polishing process ceases to occur.
Note that there is an optimum amount of polishing slurry that will cause the desired grinding or polishing interaction between the polishing media, polishing agent and the articles. If the polishing fluid level is high or low, the desired interaction will not occur and damage to the articles to be ground and/or polished can occur. The amount of polishing slurry that is the optimum amount of polishing slurry will depend on the particular articles to be polished and the type of polishing slurry and/or polishing media used.
One application in which vibratory polishing and grinding systems are used is in the grinding and polishing of carbon or carbon coated heart valve orifice rings or leaflets. Typically, such heart valve parts are formed using a pyrolytic carbon coating device such as that shown by U.S. Pat. No. 4,546,012 to Brooks, incorporated herein by reference. After being formed, the parts are ready for grinding and polishing to remove excess carbon deposits. Vibratory grinding and polishing systems can effectively be used to perform these grinding and polishing functions. As with other vibratory polishing and grinding applications, the most effective results are achieved when fresh polishing slurry is continuously added to the container and spent polishing slurry (containing pyrocarbon fines) is continuously removed from the container during a grinding or polishing cycle or run. However, because the heart valve parts may contain many small interior surfaces, the grinding or polishing must be done over a period of time, e.g., between six and twenty hours, to allow small-sized polishing media (on the order of thousandths of an inch in diameter) to adequately grind or polish small inside corners of the parts. If systems such as those taught by Dwyer et al. and Roberts were used for heart valve grinding and polishing operations, close attendance would be needed.
Another problem encountered in attempting to control the amount of admixture in the container of a vibratory polishing and grinding system is the intense vibrations (on the order of 2700 to 6000 rpm) to which the container, and any control or measurement means in mechanical contact therewith, are subjected during a typical grinding or polishing run. Such vibrations can cause mechanical damage to and premature failure of most control or measurement means known in the art. Furthermore, even if such vibrations do not cause the failure of control or measurement means, they are likely to cause inaccurate responses by the control or measurement means. Disadvantageously, whenever the control or measurements means are damaged, fail or produce inaccurate results, there is a significant possibility that overfilling or underfilling of the container will occur.
Therefore, improvements in the control of such systems are desired.